Fuse and improved indicating device therefor



Jan. 10, 1939. R. s. BENNETT FUSE AND IMPROVED INDICATING DEVICE THEREFOR Filed May 25, 1937 .C e T] on |C m v m Y b Attorney.

Patented Jan. 10, 1939 FUSE AND IMPROVED INDICATING DEVICE THEREFOR Ralph S. Bennett, Pittsfield, Mass, assignor to General Electric Company, a corporation of New York Application May 25, 1937, Serial No. 144,660

6 Claims. (Cl. 200-121) UNITED STATES PATENT OFFICE The present invention relates to fuses and has for an object the provision in a fuse of an improved indicating device which is simple and easy to assemble and which is reliable and positive in operation on both low current overloads and high short-circuit currents.

More specifically, an object of the invention is the provision in a fuse employing an indicating device of the type having a movable indicating element and a strain wire for maintaining the said element normally in a non-indicating position, of an improved arrangement of the strain wire which reduces to a minimumv or entirely eliminates the calibration difficulties and danger of fuse failure heretofore incident to the employment of similar forms of indicating means.

Other objects and the details of that which I believe to be novel and my invention will become apparent from the following description and the claims appended thereto taken in conjunction with the accompanying drawing in which Fig. 1 illustrates in longitudinal cross-section an exemplary form of fuse incorporating the improved indicating device of the present invention; Fig. 2 shows a transverse cross-section along the line 22 of Fig. 1, and Fig. 3 illus trates more clearly the arrangement of certain of the parts of the fuse of Fig. 1.

In the design of fuses, particularly of the type adapted for relatively high voltage service, a certain minimum length of fusible conductor is required for a given voltage between conductor terminals in order to prevent fuse failure due to establishment of a continuing arc along the course of the conductor upon fuse operation. In order to accommodate the required length of conductor in a casing the size of which is limited by space restrictions, it has been the practice to employ a conductor of considerably greater length than the distance 'between the fuse terminals and to support the conductor within the casing in various ways so that it does not extend directly between the terminals but follows a devious course. I have found it highly desirable in the design of such fuses to employ an indicating device which includes a wire arranged to shunt the current flow from the path of a portion of the length of the fusible conductor upon rupture of that portion due to excessive current and to arrange the indicator wire in such manner than upon fuse operation it will not provide a shorter arc path than that provided along the course of the portion of the conductor which the indicator wire is arranged to shunt.

If the indicator wire is extended from terminal to terminal of the fuse casing, the arc path provided along the course thereof upon fuse operation is considerably shorter than the arc path provided along the devious course of the main fusible conductor. This results in there being established along the course of the indicator wire upon fuse operation such a high voltage gradient that there is danger cf formation of a continuing arc with resultant failure of the fuse properly to interrupt the current flow. Such improper operation is avoided with an indicating device arranged in accordance with the present invention and wherein the indicator wire extends from one of the fuse terminals to an anchoring point remote from the other of the terminals and is arranged to shunt the current flow from the path of a portion only of the conductor upon rupture of that portion due to excessive current.

As an example of one form of fuse to which the indicating device of the present invention may be applied to advantage, there is illustrated in the drawing a fuse of the type disclosed in the copending application of Sidney R. Smith, Serial No. 144,652, filed May 25, 1937, which is assigned to the assignee of the present invention. However, as will become apparent hereinafter, my improved indicating device is applicable also for use with other forms of enclosed fuses such as, for example, those disclosed in the copending application of Sidney R. Smith, Serial No. 144,651, filed May 25, 1937, which also is assigned to the assignee of the present invention. The illustrated fuse embodies an elongated tubular enclosing casing III of suitable insulating material in which are disposed three tubes II, 12, and I3, also of insulating material, which provide compartments extending longitudinally of the casing and in close parallel side by side relationship with each other. On the opposite ends of the casing are mounted terminals caps l4 and I5 respectively, which terminal caps are secured to the casing in any suitable manner as, for example, by crimping or otherwise pressing the caps into grooves provided in the outer surface of the casing, as indicated at l6. These terminal caps II and I5 are provided with axially extending cylindrical bosses I I and II! respectively to provide for mounting of the fuse in 'commonly employed forms of terminal clips (not shown); Openings 20 are provided in the terminal caps for the venting of gases from the interior of the casing and porous disks 7| are placed intermediate the ends of the inner tubes and the terminal caps to prevent the escape of arc-quenching filling material from the interior of the casing through the openings 20. A fusible conductor 22 is threaded back and forth successively through the compartments provided by the inner tubes II, l2, and i3 and the opposite ends of this conductor are bent outwardly around the disks 2|, as indicated at 23, and then are brought out through openings 20 in the respective terminal caps l4 and I5 and fastened in any suitable manner as for example by soldering to the outer surfaces of the terminal caps, as indicated at 24. The conductor is led from tube II to tube I2 through openings 25 and from tube l2 to tube l3 through openings 26, which openings are located remotely from the terminalcaps at the ends of the casing for the principal purpose, as set forth in the previously referred to Smith application, Serial No. 144,651, filed May 25, 1937, of preventing shortcircuiting arc-over between the terminal caps and the courses of the conductor. In the illustrated fuse, tube I3 is filled with a material 21 such for example as a calcium or boric acid compound, which is capable of producing arcquenching gases upon being subjected to the heat of an arc and vent openings 28 are provided in the side walls of tube i3, while tubes II and i2 are filled with a finely divided inert arc-quenching material 29 such as sand or pulverized quartz, all as fully described and for the purposes set forth in the Smith application Serial No. 144,652. However, if desired, all of the tubes may be filled with inert material or all of them may be filled with gas-producing, arc-quenching material, depending upon the particular type of current interrupting action desired.

The indicating device of the present invention comprises, as will be seen from the drawing, a spring operated plunger 30 longitudinally disposed within cylindrical mounting boss II on terminal cap l4 and arranged to be held normally in non-indicating position by means of a strain wire 3| which preferably is of a high resistance metal and which extends in substantially parallel relation with that section of fusible conductor 22 which lies in tube l3. Plunger 38 is arranged to be operated outwardly through an opening 32 in the end of mounting boss I! by means of a spring 33 which is coiled about the plunger and which acts between disk 2| and a shoulder which forms part of a metal sleeve 34 on the inner end of the plunger. Any bright color may be applied to the outer end of the plunger to make it more readily observable when it projects through opening 32 beyond the end of mounting boss IT. The outer end of strain wire 3| 'is fastened in an opening 35 in the base of the plunger and the metal sleeve 3-". and is arranged in electrically conductive relation with terminal cap l4 through contact of spring 33 with the terminal cap. The inner end of the strain wire is threaded through the opening 26 in tube l3 through which fusible conductor 22 extends and then is threaded through openings 36, closely adjacent to opening 26, and tied so as to anchor the strain wire securely to tube l3. Thus, due to the fact that the fusible conductor itself is entirely relieved of any strain such as would be imposed by spring 33 if the strain wire were physically anchored directly to the conductor, the calibration of the fusible conductor is rendered easier. Also, there is insured greater consistency and rehability of operation of the conductor, and danger of mechanical rupture of the conductor due to strain is eliminated. With the arrangement described, the strain wire is in contiguity, that is, in contact or in close proximity, with the fusible conductor at opening 26 so that, upon rupture of the conductor, electric connection of the strain wire in the circuit, in parallel with the path of the section of the conductor in'tube i3, is effected, either through contact of the strain wire with the conductor or by means of an are drawn between the strain wire and a remaining end of the ruptured conductor. In order to insure rupture, upon fuse operation, of that section of the fusible conductor which extends through tube 13 and which parallels the strain wire, provision is made for a so-called hot spot at some point in this section. This may be accomplished in any-well known manner, as for example by a reduction in cross-section of this section of the conductor by provision of a small hole therein, as indicated at 31.

With the arrangement just described, relatively small overloads will produce fusion and volatilization of at least that section of the fusible conductor lying in tube l3 and embedded in the gas-producing filler. As a result of such fusion and volatilization of the section of the conductor lying in tube I3, the current fiow transfers to a the strain wire, either due to contact of the strain wire with the remainder of the conductor due to the passage of an are between the remaining end of the ruptured conductor and the strain wire, and the strain wire melts and releases plunger 30 which is moved outwardly to its indicating position by spring 33. Upon subjection of the fuse to currents of large value, such as occur upon short-circuit, practically all portions of the fusible conductor as well as the strain wire are dissipated substantially simultaneously and the plunger is released in thesame manner as set forth above. Thus, there is provided an indicator which is reliable and positive in operation on low current overload as well as on high current overload and short-circuit.

In the illustrated arrangement, the indicator strain wire is of substantially the same length as the section of the main fusible conductor with which the wire is arranged in parallel so that upon fuse operation the voltage gradients established along the courses of the strain wire and the corresponding section of the fusible conductor are practically the same. Thus, the arrangement of the indicating device of the present invention has the further important advantage that it eliminates danger of failure or improper operation of the fuse due to establishment of a continuing arc along the course of the strain wire when the latter ruptures due to flow of excessive current therethrough.

If in the practice of the present invention care is exercised to anchor the inner end of the strain wire in such manner that it is not in direct contact with the fusible conductor, but is in close proximity thereto, an added advantage is secured in that the strain wire normally does not shunt any portion of the current flow from the fusible conductor. As a result of this expedient, calibration of the fusible conductor is not com plicated by reason of the employment of the strain wire. Moreover, with the strain wire arranged in close proximity to the fusible conductor, but not in direct contact therewith, there is no opportunityv for a damaged fuse continuing to exist without indication thereof since any overload, no matter how small, which is capable of causing rupture of the fusible conductor itself will of necessity produce rupture of the strain wire due to the striking of an are between a rcmaining end of the ruptured fusible conductor and the strain wire. Such an arc upon striking the strain wire will produce rupture thereof even though the actual current flow is of such small value or such short duration as to be incapable of raising the whole of the strain Wire to its required fusing temperature.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In a fuse device, a casing having spaced apart terminals, a fusible conductor of greater length than the distance between said terminals, means supporting said conductor within said casing with the conductor extending by a devious course between said terminals, means electrically connecting the terminals respectively with the opposite ends of the conductor, an indicating element movable from a non-indicating position to an indicating position, a strain wire mechanically connected for maintaining said element normally in non-indicating position, said wire being arranged to shunt the current flow from a portion only of the length of said conductor upon rupture of said portion due to excessive current, and means for effecting movement of said indicating element to indicating position upon rupture of said wire.

2. In a fuse device, a casing having spaced apart terminals, a fusible conductor of greater length than the distance between said terminals, means supporting said conductor within said casing with the conductor extending by a devious course between said terminals, means electrically connecting the terminals respectively with the opposite ends of the conductor, an indicating element movably supported and biased to an indicating position, a strain wire mechanically connected for maintaining said indicating element in a non-indicating position and having one end electrically connected with one of said terminals, and means other than said conductor for anchoring the other end of said wire at a point remote from the other of said terminals and for maintaining a portion of the wire in contiguity with the conductor at a point intermediate the ends of the conductor.

3. In a fuse device, a tubular casing having terminals at opposite ends thereof, means providing a plurality of longitudinally extending compartments within the casing, a fusible conductor electrically connected between said terminals and threaded successively through said compartments, an indicating element-carried by said casing and biased to an indicating position, and a strain wire mechanically connected for maintaining said element normally in a nonindicating position, one end of said strain wire being electrically connected with the terminal at one end of the casing and the strain wire being arranged in contiguity with the section of the conductor traversing the first of the successive compartments at a point spaced substantially from said one end of the casing, the other end of said strain wire being anchored at a point spaced substantially from the terminal at the other end of said casing.

4. In a fuse device, a tubular casing having terminals at opposite ends thereof, a plurality of tubes extending longitudinally within the casing, a fusible conductor connected to the terminal at one end of said casing and extending therefrom first through one of said tubes to an opening therein and thence successively through the others of the tubes to the terminal at the other end of the casing, an indicating element mounted on said one end of the casing and biased to move to an indicating position, and a strain wire connected for maintaining said indicating element normally retracted in a non-indicating position, one end of said wire being led through said opening in said one tube in contiguity with the conductor passing therethrough and being fastened to said one tube, and the other end of said wire being mechanically connected to said indicating element and electrically connected to the terminal at said one end of the casing.

5. In a fuse device, a tubular casing, terminals carried thereby including a terminal cap mounted on one end of the casing and having a hollow tubular boss projecting from the end thereof, a fusible conductor disposed within said casing and connected between said terminals, a plunger slidable in said boss and spring biased to move outwardly through an opening in the end of the boss to an indicating position, and a strain wire for maintaining said plunger normally retracted in a non-indicating position, one end of said wire being fastened to said plunger and the other end of said wire being anchored within said casing, said wire being arranged to release said plunger upon fuse operation.

6. In a fuse device, an insulating housing, a fusible conductor therein, terminals carried by said housing and electrically connected to said conductor, and means for giving indication of fuse operation comprising an element biased to an indicating position and a strain wire connected for maintaining said element normally in a non-indicating position, said wire having one end secured to said element and its other end anchored to a wall of said housing and said wire being arranged to release said element in response to rupture of said conductor due to excessive current.

RALPH S. BENNE'I'I. 

